Fabrication of a new generation of cryogenic fixed points (triple points) in cells and their characterisation

Project Description

The recent introduction of a mutual recognition system between metrological laboratories, including the primary ones of the EU countries, has prompted a growing attention to the advantages of a longer time-spacing between the key comparisons which are the aim tool of th recognition. In the cryogenic range of the ITS-90 down to 25,4 K, one basic method allow a broadening of the verification interval of standards (10-15 years between intercomparisons, instead of 3-5 years) is a permanent realisation of the fixed points which form the basis of the ITS-90 definition, namely the triple points of argon, oxygen, neon and e-hydrogen. Their realisation with the sealed-cell technique has proved today to provide reference standards stable over a period of over 20 years.

In addition, studies are continuing on possible future improvements of the ITS-90, namely on accurate and reliable realisations of the triple point of e-deuterium, and so useful seconderay fixed point like carbon dioxide and nitrogen.

In the frame of this 24 months project it is intended, with technological co-operation being provided to IMGC and BNM-INM by other participants :

a) to produce at IMGC a new generation of sealed cells, each containing one the following gases : e-H2, e-D2, Ne, N2, O2, Ar and CO2.

b) to produce at IMGC and BNM-INM, a new generation of improved multicompartement sealed cells made of an assembly where the chambers for each gas can be independently mounted. (co-ordinator for the BNM-INM cells : Y. Hermier)

c) to supply cells to ather laboratories, according to their requests and on the basis of bilateral agreements, and to include them in bilateral comparisons.

Final Report 2005-03-18

The project started with the fabrication by IMGC of several single-substance cells, which was provided, with different type of agreements, to most of the participants.

Then from 2000 a European RTD Project under FP5 was funded, ended in middle 2003, to which participated: IMGC (Co-ordinator), BNM-INM, INTiBS, NMi-VSL, NPL and PTB, plus an Italian SME, DG. The project had also informal collaborations with NRC (Canada), NIST (USA) and NMIJ (Japan).

Different models of multiple-substance cells with demountable and interchangeable chambers in a copper block have been developed. It has been a very successful project, also according to the EU Commission and EUROMET, which reduced the uncertainty of the current triple point realisations of e-H2, Ne, O2 and Ar by a factor of at least three.

In addition, the reason for the larger dispersion of the results with e-H2 and Ne where understood to arise from the variable isotopic composition of the gas commercially available. Studies were made for the first now finalised, while for the second they started during the EU Project and now are continuing under EUROMET project 770.

A new type of calorimeter, specialized for fixed point realisations –but usable also for thermometer inter-comparisons– between 13 K to room temperature was developed, based on a two stage closed-cycle refrigerator, allowing to achieve the top accuracy. It is becoming commercially available and four copies of it will be run in participant laboratories. It can be extended in the future with a better refrigerator down to 4 K –and below.

A copious amount of scientific papers has been published on the relevant outcomes of the this project and new and better procedures developed for the cryogenic fixed point realisations, which can be used also for improving the contents of the CCT documents on Supplementary Information (WG1) and on uncertainty of ITS-90 realisation techniques (WG3).

Further Information

NIST, NRC and KRISS will also join the project - Part of these cells will also be considered by the PTB co-ordinator for participation in the Euromet agreed project n°377

Thermometry (T)
Franco Pavese, INRIM (Italy)
Phone: +39 011 39 77 341
Fax: +39 011 39 77 349
E-Mail: f.pavese@imgc.to.cnr.it
Coordinating Institute
INRIM (Italy)